1. Field of the Invention
The present invention relates to a method for preparing a conjugated diene polymer by a vapor phase polymerization. The method reduces the amount of hydrocarbon compounds used as polymerization solvents which are discharged into the environment and improves dispersion of a rubber reinforcing agent in the resulting polymer, thereby producing a conjugated diene polymer product with improved abrasion resistance, durability, and rupture resistance.
2. Description of the Background Art
High cis-polybutadiene products containing a large number of cis-1,4-units have long been manufactured on an industrial scale and have been widely used for tires and other rubber products. In particular, polybutadiene rubbers are used for tires to a great extent taking advantage of their excellent characteristics such as high abrasion resistance, high impact resilience, superb low temperature properties, favorable flex resistance, and low rolling resistance.
In recent years, there has been a strong desire for more excellent fracture characteristics and abrasion resistance in the materials for tires, to ensure a lower fuel consumption and to decrease the weight of vehicles without impairing safety. Generally, these tire characteristics depend on the rubber material used. A method of dispersing fillers sufficiently in rubber by using an improved method of kneading and blending the raw material rubber and the fillers has been known as a means for improving the fracture characteristics and abrasion resistance of polybutadiene. This method, however, does not bring about satisfactory results.
Heretofore, polymerization for the manufacture of polybutadiene has been carried out in a liquid phase using a catalyst. An example of a catalyst system which is effective for manufacturing a high cis-polybutadiene has been disclosed in European Patent No. 11184. The catalyst system disclosed in this patent is used for solution polymerization of butadiene. This catalyst comprises a carboxylate of a rare earth element, trialkylaluminum and/or hydrogenated alkyl aluminum, and another Lewis acid. In solution polymerization of a conjugated diene compound, unreacted monomers and the solvent must be removed from the polymer produced. Low molecular weight compounds which are separated from the polymer products must be reclaimed to prevent these compounds from being released to an environment together with exhaust gas and drainage.
A method of polymerizing a liquid conjugated diene compound without using a solvent is also known. This method, however, involves difficulty in controlling the polymerization reaction. In addition, the polymerization is significantly exothermic, which is also accompanied by risks in controlling the reaction. Also, the monomers must be separated from the polymers produced in the same manner as in solution polymerization, which also give rise to environmental problems.
In recent years, vapor phase polymerization has proven to be effective for the manufacture of polyethylene and polypropylene and has been utilized in industrial scale manufacture of these polymers. Vapor phase polymerization which does not use any solvents is advantageous for environmental conservation due to a reduced amount of low molecular weight compounds discharged into air and drainage.
The Ziegler-Natta catalyst based on a transition metal or a rare earth element to be used in solution polymerization of conjugated diene compounds, which is disclosed in European Patent No. 11184, for example, can maintain its activity only for a very short period of time during vapor phase polymerization. Only a slight amount of polymer can be produced using this catalyst. In addition, the polymer produced adheres to the wall of the vessel due to its adhesive properties, giving rise to poor productivity.
As a means to overcome these problems, Japanese Patent Application Laid-open No. 165811/1995 disclosed a process for vapor phase polymerization of butadiene using a catalyst comprising a rare earth element carried on inert inorganic particles such as silica. WO No. 96/04322 and WO No.96/04323 proposed a process for vapor phase polymerization using a catalyst system containing a rare earth element or a transition metal supported on a carrier or in the form of a solution together with a co-catalyst in the presence of inert inorganic solid particles such as carbon black. However, all these catalysts exhibit an activity much lower than that of a conventional catalyst system based on a rare earth metal used in solution polymerization, for instance. In addition, the process of the WO No. 96/04323, for instance, cannot adequately control the molecular weight due to direct injection of an undiluted or high concentration solution of aluminum hydride to the reactor.
In view of this situation, the present inventors have conducted extensive studies and found that a catalyst system using a rare earth metal compound is effective for vapor phase polymerization of conjugated dienes, if utilized together with carbon black used as a rubber reinforcing agent and an inorganic filler such as silica. Using this method dispersion of the rubber enforcement agent in the polymer can be improved and the molecular weight of the conjugated diene polymer can be controlled with ease.